Numerous types of fluid filtration systems, e.g. such as those used for home water filtration and respirators, are commercially available. Nonwoven fibrous webs are frequently used as a filtration medium in such fluid filtration systems. Such nonwoven fibrous webs may include two or more kinds of fibers, for example, two different populations of microfibers, each having a different average diameter, so that the nonwoven fibrous web can filter particles of a broad range of sizes. Generally the different populations of fibers are intermixed within a single-layer web.
One example of a multi-layer nonwoven fibrous web useful as a filtration medium is described in U.S. Patent Application Publication No. US 2004/0035095 (Healey). Another example of a multi-layer nonwoven fibrous web is provided by so-called SMS (Spunbond-Meltblown-Spunbond) webs comprising a layer of spunbond fibers, a layer of meltblown microfibers, and another layer of spunbond fibers. In some nonwoven fibrous webs useful as a filtration medium, it has been found to be advantageous to incorporate sorbent particulates, such as activated carbon, within the web.